This invention relates to exhaust gas clean-up catalysts for cleaning the exhaust gas discharged from fuel combustion engines, for example internal combustion engines and external combustion engines, and to methods of exhaust gas clean-up using the catalysts
Three-way catalysts capable of simultaneously reducing the levels of hydrocarbon (HC), nitrogen oxides (NOx) and carbon monoxide (CO) have been under develop for practical application as catalysts for exhaust gas clean-up since the 1970s. Such catalyst are described for example in Japan Patent Kokai No. 60-54730 and Japan Patent Kokai No. 61-11147. A three-way catalyst converts the reducing components (HC, CO, H2) and oxidising components (NOx, O2) in exhaust gas to harmless components (H2O, CO2, N2) by redox relation at a near theoretical fuel-air ration (xcexxcx9c1).
At the relatively low temperatures prevailing timely after start-up of a fuel combustion engine, however, even a three-way catalyst has low activity, making it difficult to clean up the noxious components from the exhaust gas. As a method of quickly raising the activity of three-way catalysts, therefore, in-line catalyst, systems combining in-line catalysts and underfloor catalysts, electrically heated catalysts, burner heated catalysts and adsorbents have heretofore been used, but for reasons including catalyst location, increase in cost, reduction in fuel consumption, and durability, the problem still remains to be solved
Catalysts with low temperature activity have been under development that have high catalytic capability enabling effective clean-up of exhaust gas in the low temperature region directly after start-up of fuel combustion engines without recourse to the aforementioned indirect devices or techniques of raising the active capability of the catalyst. For example, catalysts whereby the nitrogen oxides (NOx) in exhaust gas can be cleaned up at low temperature are disclosed in the specifications of Japan Patent Kokai No. 6-190276, Japan Patent Kokai Nol. 6-198178 and Japan Patent Kokai No. 6-262079. Similarly, catalysts capable of cleaning up hydrocarbons (HC) and carbon monoxide (CO) at low temperature are disclosed in Japan Patent Kokai No. 7-204510.
Although these prior art catalysts have been improved or modified to meet the aim of cleaning up exhaust gas, room for improvement exists in that they have low oxidising activity is respect of saturated hydrocarbons; in particular, they have not afforded sufficient clean-up capability at low temperature prevailing directly after start-up of fuel combustion engines
The inventors have found that catalysts containing specified elements and compounds were adequately capable of cleaning up exhaust gas at the low temperatures prevailing directly after start-up of fuel combustion engines. In particular, the inventors discovered catalysts with low temperatures activity that afforded outstanding clam-up of the hydrocarbons, especially the saturated hydrocarbons, in the exhaust gas evolved in the low temperature region directly after start-up of fuel combustion engines.
The aim of the invention is accordingly to provide catalysts with low temperature activity whereby the exhaust gas evolved in the low temperature region directly after start-up of fuel combustion engines can be satisfactorily cleaned up.
The exhaust gas clean-up catalysts used in the invention to clean the exhaust gas from fuel combustion engines are catalysts containing one or two or more noble metals chosen from the group platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), osmium (Os) and other noble metals and one or two or more metals or metal oxides chosen from the group tungsten (W), niobium (Nb) and molybdenum (Mo) oxides.
The exhaust gas clean-up catalysts in the invention can also contain the metal oxide(s) ceria and/or zirconia
The invention further comprises exhaust gas clean-up catalyst wherein the aforesaid catalysts are supported on one or two or more metal oxides chosen from the group alumina silica, titania and zirconia
The catalyst compositions in the invention use at least one of tungsten (W) metal or tungsten oxide.
In the preferred embodiment of the invention, the catalysts also contain at least one of niobium (Nb) metal, niobium oxide, molybdenum (Mo) metal or molybdenum oxide.
On inclusion of these oxides in alumina or zirconia, catalyst acid increases, promoting dissociative adsorption of hydrocarbons, and it may be supposed that the oxides contribute to the oxidation of hydrocarbons as a result thereof. Development of acidity is especially prominent with tungsten oxide because it complexes with alumina and zirconia, and tungsten oxide is therefore especially preferred.
In the preferred embodiment of the invention, the content of tungsten (W), niobium (Nb) and molybdenum (Mo) in the catalyst is in the approximate range 0.5-20 wt %, preferably 1.5-15 wt %, as the amount of metal with respect to the whole catalyst. With a loading in the approximate range 0.5-20 wt %, catalyst efficacy can be fully displayed and the problem of saturation of the additive effect does not arise.
The metal oxide(s) ceria and/or zirconia can be used in the catalyst compositions of the invention.
Ceria has oxygen storage capability and can stabilise catalyst performance when deviations in fuel/air ratio occur close to the theoretical fuel/air ratio. Ceria can also confer durability on the noble metals comprising the catalytic composition
Zirconia can increase the heat-resistance of tungsten (W), niobium (Nb) and molybdenum (Mo) as well as that of ceria. Moreover, acidity can be father raised by complexing with tungsten, etc., and we expect this strong acidity to afford improvement in the heat resistance of noble metals.
In the preferred embodiment of the invention, exhaust gas clean-up catalysts wherein the metal oxide(s) ceria and/or zirconia and the one or two or more metal oxides chosen from the group tungsten (w), niobium (Nb) and molybdenum (Mo) oxides consume a compound oxide are preferred.
Catalysts of preferred heat resistance can be formulated by constituting a compound oxide. Although the actual structure and action of the aforesaid compound oxide are not fully understood, the inventors suppose that in a compound oxide of tungsten oxide and zirconia, for example, the crystallites of tungsten oxide are mutually isolated by zirconia crystallites, hindering sintering and thereby improving heat-resistance.
In a preferred embodiment of the invention, a compound oxide of ceria, a compound oxide of a ceria-zirconium and preferably a compound oxide of zirconia, with the oxide(s) of tungsten (W), niobium (Nb) and molybdenum (Mo) is used in the catalyst composition.
Compound oxides wherein zirconia has been added to oxides of tungsten (W), niobium (Nb) and molybdenum (Mo) have excellent capacity for cleaning saturated hydrocarbons from exhaust gas by oxidation, especially under relatively low temperature conditions. In particular, significant low temperature activity is displayed under conditions where the carbon monoxide concentration in the exhaust gas is 0.5% or less.
The noble metals have long been used as metals constituting three-way catalysts. In the catalyst compositions of the invention a single metal or a mixture of two or more of the metals can be used. In the preferred embodiment of the invention, the use of platinum (Pt) is preferred. For a noble metal, platinum (Pt) has outstanding saturated hydrocarbon clean-up capability under low temperature conditions.
One or two or more metal oxides chosen from the group alumina, silica, titania and zirconium and their mixtures can be used as the support in the catalyst compositions of the invention. The compositions so constituted are used both to stabilise the active components of the catalyst and to raise clean-up performance by increasing the area of contact with the exhaust gas. In the preferred embodiment of the invention, it is preferred to use alumina as the support.
Although the exhaust gas clean-up catalysts of the invention require the use of the aforesaid catalyst compositions, the catalysts may contain other optional components within the limits of adherence to the spirit of the invention.
According to the preferred embodiment of the invention, the exhaust gas clean-up catalyst can be produced using conventional catalyst manufacturing technology, suitably by adding optional components as necessary to the catalyst components (wash coat) containing tungsten oxide and ceria and/or zirconia, and then supporting platinum thereon.
Any method may be used to add tungsten oxide. Moreover, the order of addition is optional; for example, the addition maybe made at the same time as, before, or after the addition of platinum, the addition may be made along with the alumina and/or zirconia, or the addition may be made after complexing tungsten oxide with zirconia.
According to the preferred embodiment of the invention, the exhaust gas clean-up catalyst can be produced by forming the catalyst composition as an oxide solely by calcination in air, without recourse to reductive treatment at any stage of catalyst preparation. The salts or compounds of tungsten and zirconium used in catalyst production are thermally decomposed by calcination in air, facilitating uniform mixing with other catalyst components in the course of combination with oxygen in the air. Moreover, the production flowchart can be greatly simplified since calcination in special atmospheres other than air, for reductive treatment or the like, is unnecessary.
According to the preferred embodiment of the invention, a method of exhaust gas clean-up and exhaust gas clean-up apparatus are preferred wherein the exhaust gas is treated in the channel through which the which the exhaust gas from the fuel combustion engine discharges, first with an exhaust gas clean-up catalyst excellent for oxidation of carbon monoxide and unsaturated hydrocarbons, disposed within the exhaust gas channel, and then with the exhaust gas clean-up catalysts claimed for the invention, disposed at the same location as or rearward of the location of the said catalyst.
By pretreating the carbon monoxide and unsaturated hydrocarbons present in the exhaust gas with an exhaust gas clean-up catalyst excellent for oxidation, it is possible to prevent the negative effects of a high concentration of carbon monoxide, for example poisoning of platinum metals; the saturated hydrocarbons can then be effectively cleaned up in the low temperature region by means of an exhaust gas clean-up catalyst of the invention located rearward.
A catalyst containing one or more metals chosen from the group comprising palladium, rhodium, together with ceria may be cited as an example of the exhaust gas clean-up catalyst excellent for oxidation of carbon monoxide and unsaturated hydrocarbons that is used in the invention.
According to the preferred embodiment of the invention, it is preferred to use an exhaust gas clean-up catalyst wherein, to prevent the negative effects of carbon monoxide, the exhaust gas clean-up catalyst of the invention is supported in a lower layer or upper layer, and an exhaust gas clean-up catalyst with a high capability for removal of carbon monoxide and hydrocarbons is supported in the upper layer or lower layer on the same support (for example a honeycomb support), or a mixture thereof.
The catalyst is then able to remove carbon monoxide from exhaust gas with high efficiency, while the saturated hydrocarbons in the exhaust gas can be removed efficiently and effectively with the gas clean-up catalyst of the invention disposed at the same location or located rearward.
The exhaust gas clean-up apparatus of the invention comprises apparatus for cleaning up the exhaust gas from fuel combustion engines wherein the exhaust gas clean-up catalyst of the invention and, if necessary, an exhaust gas clean-up catalyst that removes carbon monoxide and unsaturated hydrocarbons by oxidation are disposed in the path of the exhaust gas outflow. As a practical embodiment of the invention, the exhaust gas clean-up catalyst of the invention can be used as an in-line catalyst, in-line/underfloor catalyst, electrically heated catalyst or burner catalyst for reducing the hydrocarbon level of the exhaust gas at low temperature. It is also possible to use a combination of hydrocarbon adsorbents, etc., and the aforesaid forms of catalyst.
xe2x80x9cFuel combustion enginexe2x80x9d here means an engine that generates energy by burning fuel, irrespective of whether described as an internal combustion engine, an external combustion engine or by another term xe2x80x9cExhaust gasxe2x80x9d means the gas formed by the fuel combustion engine after fuel combustion and includes incompletely burned or unburned fuel gas. xe2x80x9cSaturated hydrocarbonsxe2x80x9d denotes hydrocarbons that do not have double bonds between carbon atoms. The term includes, for example, ethane, propane and isobutane.